Magnetophoresis relates to the movement of an object under the effect of an inhomogeneous magnetic field.
It is currently used for handling operations such as trapping, separation, mixing and transportation of objects, including, for example, biological species functionalized by magnetic nanoparticles or microparticles.
The magnetophoretic force acting on a magnetic particle is given by the expression:Fm=Mp∇B  (1)
where Mp is the magnetic moment of the particle, proportional to its volume, and ∇B is the magnetic field gradient in which the particle is placed.
According to equation (1), the magnetic force exerted on an object placed in a homogeneous magnetic field is zero, independently from the value of the magnetic field, however high it is.
Consequently, one of the necessary conditions for generating significant magnetic forces is the existence of a magnetic field gradient, i.e. the existence of a local inhomogeneity of the magnetic field in space.
The aim is therefore to generate the strongest possible magnetic field gradients, and to do so on a sub-millimetric scale.
Microdevices mainly developed for the control and/or handling of ferromagnetic or superparamagnetic objects currently use microcoils or soft materials coupled to an external magnetic field, possibly in association.
The first type of microdevice is based on the use of microcoils typically implemented by conventional micromanufacturing techniques such as photolithography.
However, these microcoils suffer from three major disadvantages.
On the one hand, the generated magnetic fields are limited by the heating of the circuit.
In fact, the magnetic field generated by a coil with a resistance R is directly proportional to the intensity I of the current flowing through it.
The flow of a current of intensity I, during a time t, in a circuit causes heating through the Joule effect (RI2×t), which inevitably results in a limitation of the current and therefore a limitation of the generated magnetic field.
By way of example, the continuous current in a copper conductor with a 100 μm2 cross section is in the order of 10−4 A.
The magnetic field of a single coil with a 10 μm radius of such a conductor is in the order of 0.1 mT and the maximum magnetic field gradient is in the order of 102 T/m.
Fed with pulsed current, the microcoils can generate much higher fields than with continuous current, typically 1000 times greater, but during a time which is often less than one millisecond, which is unsuitable for the intended applications.
Furthermore, these microcoils always require an external current feed for their operation.
The second type of microdevice combines the use of soft magnetic materials with an external macroscopic magnetic field.
Placed in an external magnetic field, the soft magnetic material becomes magnetized and then behaves in a manner similar to a permanent magnet as a field source.
The soft materials for magnetic field micro-sources are implemented by micro/nanomanufacturing techniques.
The magnetic field sources obtained have the size of the patterns of the implemented soft materials, with a sub-millimetric dimension.
Placed in an external macroscopic magnetic field, these sources produce strong magnetic fields and field gradients, substantially modulated on the scale of the patterns.
The use of a variable and switchable external magnetic field renders these magnetic sources variable and switchable.
Finally, the manufacturing of devices comprising a film made from a hard magnetic material deposited on a silicon substrate, structured either topographically by using micromanufacturing techniques [Walther09], or thermomagnetically [Dumas-Bouchiat10], in such a way as to form a plurality of micromagnets, has recently been described.
These devices offer the advantage of being autonomous, since, once magnetized, they require neither an energy source nor an external magnetic field source.
Furthermore, the micromagnets thus formed produce strong magnetic field gradients, up to 106 T/m.
However, the manufacturing of these different devices is costly, since it requires leading-edge technologies, and is not therefore suitable for the production of large series of low-cost devices.
In particular, techniques based on silicon substrates are limited by the available sizes of these substrates and therefore face the impossibility of forming devices with large dimensions.
Furthermore, a certain flexibility of the devices would be desirable for some applications.
Furthermore, it would be useful, for example for in vitro applications, to have transparent or more or less translucent devices in such a way as to be able to observe, through optical transmission microscopy, the behavior of particles influenced by the generated magnetic field.
The article by D. Issadore et al., “Self-Assembled magnetic filter for highly efficient immunomagnetic separation”, Lab Chip, 2011, 11, pp. 147-151, reports on the manufacturing of a polymer film comprising the placing in suspension, in polydimethylsiloxane (PDMS), of particles of NdFeB, the magnetization of said particles through the application of an intense external magnetic field, then the reticulation of the PDMS, fixing the NdFeB particles.
However, the distribution of the particles in the PDMS matrix is random.
Furthermore, each particle is isolated from the particles surrounding it, in such a way that the traps thus formed have the size of an individual particle and therefore have a limited trapping capability.
A need therefore exists for the manufacturing of a device which produces, on a submillimetric scale, a high magnetic field gradient, and which can be manufactured at a low cost.
Furthermore, according to the intended applications, this device must be able to be flexible and/or transparent.
Another object of the invention is to define a simple and economical method for producing a device capable of being magnetized, either in a permanent manner or under the action of an external magnetic field, in order to generate a substantial magnetic field gradient on a submillimetric scale.